Optical electric field sensor using optical component having electrooptical effect

ABSTRACT

An optical electric field sensor comprises optical components 2 through 4 and 11 through 13 including an optical crystal and is for measuring the intensity of an electric field, spontaneously or forcedly generated, by the use of variation of at least one of an intensity, a phase, and a polarization direction of a light beam passing through the electric field. The above-mentioned optical components are arranged and sealed in a package 7 made of at least one of a glass material such as quartz, a ceramics material, and a plastic material such as vinyl chloride having an antistatic-treated surface. More effectively, a main portion of the surface of the package 7 is subjected to abrasion. On the other hand, the optical crystal having an electrooptical effect is fixedly surrounded by a heat insulation material. The entire surface of the optical crystal substrate is coated with conductive resin. Silicone is applied in an area between modulation electrodes.

TECHNICAL FIELD

This invention relates to an optical electric field sensor for use inmeasurement of an electric field intensity within a spatial field,typically, in EMC measurement (noise measurement).

BACKGROUND ART

An optical waveguide Mach-Zehnder interferometer has a structure inwhich an optical waveguide is branched and one or both of branchedoptical waveguides are applied with an electric field parallel to acrystal axis thereof to phase-shift light beams propagating therein,which beams are thereafter combined again. Because a light intensityafter combined is varied by the electric field applied thereto, theinterferometer is used as an electric field sensor for detecting, bymeasurement of the light intensity, an electric field intensity appliedto antennas connected to electrodes. The intensity of an outgoing lightbeam of the Mach-Zehnder interferometer exhibits a trigonometricfunction wave curve with respect to the electric field applied to theelectrodes.

FIG. 1(a) shows one example of a conventional optical electric fieldsensor. As illustrated in the figure, the optical electric field sensorcomprises an optical branched waveguide type interferometer formed on anLiNbO₃ substrate by diffusion of Ti. One of two branched opticalwaveguides is provided with electrodes to form an optical modulator. Theoptical modulator is fixedly housed in a case 1 made of plastic. Theelectrodes of the optical modulator are connected to antennas 2,respectively. A polarization maintaining fiber 3 and a single mode fiber4 are connected to a light incident side and a light outgoing side ofthe optical modulator, respectively. Connectors 6 are provided at theends of fibers 3 and 4. An electric field spontaneously or forcedlygenerated is transmitted through the antennas to the electrodes toproduce phase modulation in the optical waveguide. The light beamcombined thereafter is modulated in intensity and, thus, has the lightintensity corresponding to the electric field.

FIGS. 2(a)-2(d) show a conventional optical waveguide Mach-Zehnderinterferometer used in the optical modulator illustrated in FIG. 1(a).As illustrated in FIG. 2(a), the optical waveguide Mach-Zehnderinterferometer has a structure such that an optical waveguide isbranched into branched optical waveguides 12 and 12 arranged onsubstrate 21, one or both of which are applied with an electric field 18parallel to an optical axis through modulation electrodes 22 and 22 toprovide phase-shift in the optical waveguides before being combinedagain. An input light beam is shown at 15 in FIG. 2(a), and an outputlight beam is shown at 16. Because a light intensity after combinationis varied by the electric voltage applied thereto, the interferometercan be used as an electric field sensor for detecting, by measurement ofthe light intensity, an electric field intensity applied to antennas 2as a low voltage applied across the modulation electrodes 22 and 22.

FIG. 3 shows an optical modulation characteristic of the Mach-Zehnderinterferometer illustrated in FIG. 2(a). As illustrated in FIG. 3, anoutput intensity (relative intensity) of the light beam modulated inintensity by the Mach-Zehnder interferometer varies along atrigonometric function wave (sine wave) curve with respect to theapplied voltage. In view of the above, adjustment (optical biasadjustment) is performed so that the light intensity is located at alinear variation point (a middle point between the maximum level and theminimum level) of the trigonometric function wave when the appliedvoltage is equal to 0 V. In this event, variation in light intensity andthe applied electric field exhibit a proportional relationship. It istherefore possible, as an electric field sensor, to measure the appliedelectric field by the light intensity. In other words, such acharacteristic is required for use as an electric field sensor.

The conventional optical electric field sensor, however, has a distancebetween the electrodes which is as small as several microns. If foreignsubstances, such as alkali ions, exist between the electrodes, thevoltage applied across the electrodes is accumulated as a chargedvoltage. This results in fluctuation of an optical modulation ratio withrespect to the applied voltage. Such fluctuation tends to occur in a lowfrequency rather than in a high frequency (DC drift, giving a largestinfluence upon a direct-current voltage). In that event, measurementaccuracy of the optical electric field sensor is deteriorated. When theoptical electric field sensor of this type is subjected to temperaturevariation, carrier particles are generated within a crystal, moved, andnonuniformly accumulated in the vicinity of the electrodes to produce aninternal electric field. This results in instability (temperature drift)of the outgoing light beam. Such fluctuation in characteristic is greatand small when the temperature variation is drastic and gentle,respectively. The temperature drift will briefly be described inconjunction with FIG. 1(b) and FIG. 1(c). Referring to FIG. 1(b), theoptical electric field sensor is put in a condition where an ambienttemperature is equal to 30° C. which is higher than a room temperature.An incident light beam is incident to the polarization maintaining fiber3 (FIG. 1(a)) and passes through the conventional optical electric fieldsensor to be emitted from the single mode fiber 4 as a normal outputlight beam having a waveform A. An abscissa and an ordinate represent anapplied electric field and a light intensity, respectively. Herein,adjustment is made so that the light intensity is located at a middlepoint between the maximum level and the minimum level when the electricfield applied to the antennas is equal to 0 (V). As far as a normaloperation is carried out, the waveform is as illustrated in FIG. 1(b).When subjected to the temperature drift, the output light beam emittedfrom the single mode fiber 4 has a waveform B illustrated in FIG. 1(c).In the waveform B, the light intensity is phase-shifted by π/4 withrespect to the waveform A of the incident light beam when the electricfield applied to the antennas is equal to 0 (V). Such shift is thetemperature drift which deteriorates the temperature characteristic ofthe optical electric field sensor. As a result, the sensitivity becomesunstable.

In order to improve the temperature characteristic, the opticalmodulator used in the conventional optical electric field sensor adoptsa method of indirect compensation. Specifically, the optical crystal isgiven distortion equal in magnitude and reverse in polarity to the driftby, for example, application of a physical stress caused by a Peltierelement or the like, and alternatively, addition of an extra electricfield reverse to the distortion the modulation electric field. As knownin the art, such fluctuation in characteristic can be avoided by forminga conductive film on the surface of the substrate to cancel the electriccharge within the crystal.

However, there has been no such optical electric field sensor that has astructure for suppressing heat conduction to the optical modulator,which heat conduction substantially is a cause of deterioration of thetemperature characteristic. In order to monitor the output of theoptical modulator, to measure the temperature drift, and to applydistortion for compensating it as described above, a device foroperating these mechanisms is required. Furthermore, an accuracy isrequired. In addition, a typical optical modulator uses the conductivefilm such as a semiconductor Si film to suppress the fluctuation incharacteristic. However, because sputtering or vacuum deposition isadopted therefor, there arises a problem of an increase in process time.

On the other hand, when the above-mentioned Mach-Zehnder interferometeris manufactured, the optical modulation characteristic with respect tothe applied voltage generally changes in dependence upon thecharacteristic of the LiNbO₃ substrate or the manufacturing condition ofthe element. Specifically, it is possible to assure a reproduciabilityof those characteristics such as a half-wavelength voltage and a loss.However, it is difficult to adjust the light intensity at the appliedvoltage of 0 V to the middle point between the maximum level and theminimum level as required to the electric field sensor. In view of theabove, it is a general practice to carry out adjustment (optical biasadjustment) by giving distortion to the waveguide after manufactured.

In the meanwhile, the electric field sensor has a structure in which theantennas made of metal receive the electric field to generate theapplied voltage at the electrode portions of the optical modulator. Whenany metal other than the antenna is present around the sensor, theelectric field generated around the electric field sensor is disturbed.Therefore, the package is preferably made of a nonmetallic material toremove metal components other than the antennas. Use is generally madeof resin such as plastic. The electric field sensor thus manufactured isused to measure the electric field intensity on the order of severalmV/m because of its characteristic, and is readily subjected to theinfluence of the electric field generated therearound. In addition, thepackage made of resin such as plastic generates an electrostatic fieldhaving such a level that fluctuates the optical bias. Since theelectrostatic field generated by the package is greatly concerned withvariation of humidity or the like, it is difficult to provide an elementhaving a constant optical bias. However, in order to compensate fordeviation of the optical bias due to the electrostatic field,consideration has mainly been directed to adjustment of the optical biasafter packaging.

It is therefore one object of this invention to remove an electrostaticfield generated by a package material after packaging as well as toimprove heat insulation of an optical waveguide element so as to removefluctuation of an optical bias due to temperature drift of aMach-Zehnder interferometer.

It is another object of this invention to provide an optical electricfield sensor which has a structure for suppressing heat conduction ofthermal fluctuation outside of the optical electric field sensor to anoptical crystal, to thereby dispense with the device requiring the highaccuracy and to improve a temperature characteristic.

It is still another object of this invention to provide an opticalelectric field sensor having a conductive film formed by an inexpensiveand simple process.

It is other object of this invention to provide an optical electricfield sensor which is capable of inhibiting interference with anexternal environment to readily prevent invasion of dirt or foreignsubstances by applying an agent having a stable characteristic on anarea between electrodes where invasion of the foreign substances isotherwise easy.

It is a further object of this invention to provide an optical electricfield sensor having a structure of removing an electrostatic fieldgenerated by a package material after packaging to thereby avoiddisturbance of an electric field to be measured.

SUMMARY OF THE INVENTION

According to this invention, there is provided an optical electric fieldsensor which comprises optical components including an optical crystaland which is for measuring the intensity of an electric field,spontaneously or forcedly generated, by the use of variation of at leastone of an intensity, a phase, and a polarization direction of a lightbeam passing through the electric field, wherein the optical componentsare arranged in a package made of at least one of a heat insulationmaterial and a plastic material having an antistatic-treated surface.

According to this invention, the heat insulation material preferably isat least one selected from a ceramics material and a glass material.Instead of the ceramics material, use may be made of a glass materialincluding quartz. In either event, it is preferable that a surface(including an inner surface) of the package is partially or entirelysubjected to abrasion (grinding).

According to this invention, the resin such as plastic, which hasconventionally been used, is made to have a surface conductivitysubstantially equivalent to that of a semiconductor and is used as aplastic material having an antistatic-treated surface which provides anantistatic effect. The plastic material preferably comprises vinylchloride.

According to this invention, the electric field is preferably appliedthrough antennas connected to the optical crystal.

According to this invention, it is preferable that the package isfixedly surrounded by a heat insulation material such as expandedpolystyrene foam.

According to the optical electric field sensor of this invention, theoptical components are preferably arranged in a manner such that atleast a pair of modulation electrodes are located in the vicinity of anoptical waveguide formed on an optical crystal substrate having anelectrooptical effect and an electric field spontaneously or forcedlygenerated is led to the pair of modulation electrodes.

According to this invention, there is provided an optical electric fieldsensor which comprises at least a pair of modulation electrodes locatedin the vicinity of an optical waveguide formed on an optical crystalsubstrate having an electrooptical effect and which is for measuring anelectric field intensity by the use of variation of at least one of anintensity, a phase, and a polarization direction of a light beam that iscaused by leading to the pair of modulation electrodes an electric fieldspontaneously or forcedly generated, wherein at least one of treatmentsis carried out which include application of conductive resin onto anentire surface of the optical crystal substrate and application ofsilicone between the modulation electrodes.

Specifically, according to this invention, a conductive resin film isused as the conductive film and a structure is used where the conductivefilm is applied onto the entire surface of the crystal substrate tocause migration and cancellation of unstable electric charge produced bytemperature variation. The conductive film used must be carefullyselected in respect of a resistance and a material so as not to affectthe drift. According to this invention, silicone is used. This agent isexcellent in isolation from the external environment, rapid in drying,and easy in application. In addition, the agent itself does not act as aforeign substance causing voltage fluctuation because of absence of anyundesired unstable ions. Thus, it is noted that the agent is reliablefor a long time. According to this invention, it is preferable that theoptical crystal substrate is fixedly surrounded by a heat insulationmaterial. Specifically, according to this invention, the opticalmodulator is fixedly housed in a case made of a heat insulation materialsuch as a low heat conduction material, a foam-containing material, anda low heat conduction and foam-containing material. Alternatively, acase in which the optical modulator is fixedly housed is covered with aheat insulation material. Thus, heat conduction from the outside of theoptical modulator is suppressed.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1(a) is a view illustrating a structure of a conventional opticalelectric field sensor;

FIGS. 1(b) and 1(c) are views for describing an influence of atemperature upon a relationship between a light intensity and anelectric field in the optical electric field sensor in FIG. 1(a);

FIG. 2(a) is a plan view of the optical electric field sensorillustrated in FIG. 1(a):

FIG. 2(b) shows a relationship between the time and the light intensityof an input light beam in the optical electric field sensor in FIG.2(a);

FIG. 2(c) shows a relationship between a voltage of an electric fieldsignal and the time in the optical electric field sensor in FIG. 2(a);

FIG. 2(d) shows a relationship between the time and the light intensityof an output light beam in the optical electric field sensor in FIG.2(a);

FIG. 3 shows an optical modulation characteristic with respect to anapplied voltage in the optical electric field sensor in FIG. 2(a);

FIG. 4 is a perspective view illustrating a structure of an opticalelectric field sensor according to a second embodiment of this inventionwith an upper half of a package removed;

FIG. 5(a) shows an optical electric field sensor according to a thirdembodiment of this invention;

FIGS. 5(b) and 5(c) are views for describing an influence of atemperature upon a relationship between a light intensity and anelectric field in the optical electric field sensor in FIG. 5(a);

FIG. 6(a) is a plan view of an optical electric field sensor accordingto a fourth embodiment of this invention;

FIG. 6(b) is a sectional view of the optical electric field sensor inFIG. 6(a) taken along line 6(b)--6(b) of FIG. 6(a);

FIG. 7 shows a relationship between an applied voltage and an outputlight beam in the optical electric field sensor in FIGS. 6(a) and 6(b);

FIG. 8(a) is a plan view of an optical electric field sensor accordingto a fifth embodiment of this invention;

FIG. 8(b) is a sectional view of the optical electric field sensor inFIG. 8(a) taken along line 8(b)--8(b) of FIG. 8(a);

FIG. 9 shows a relationship between an applied voltage and an outputlight beam in the optical electric field sensor illustrated in FIGS.8(a) and 8(b); and.

FIG. 10 shows an optical electric field sensor according to a sixthembodiment of this invention.

DETAILED DESCRIPTION

Now, description will be made in detail with reference to theaccompanying drawings.

First Embodiment

An optical electric field sensor according to a first embodiment of thisinvention has a structure similar to that of the conventional opticalelectric field sensor illustrated in FIG. 1(a) except that a case 1 ismade of glass instead of plastic in the conventional example.

Specifically, the optical electric field sensor comprises an opticalbranched waveguide type interferometer formed on an LiNbO₃ substrate bydiffusion of Ti. One of two branched optical waveguides is provided withelectrodes. This optical modulator is fixedly housed in the glass case.The electrodes of the optical modulator are connected to antennas 2,respectively. A polarization maintaining fiber and a single mode fiberare connected to a light incident side and a light outgoing side of theoptical modulator, respectively.

When the optical electric field sensor having the above-mentionedstructure according to the first embodiment is put in an environment of30° C. which is higher than the room temperature, no fluctuation inoptical modulation characteristic is observed.

Second Embodiment

FIG. 4 is a slightly perspective view of an optical electric fieldsensor according to a second embodiment of this invention with an upperhalf of a quartz package removed. A Mach-Zehnder interferometer patternwas formed on a Z-cut substrate 11 (having a crystal axis in a Zdirection) of LiNbO₃ by a Ti pattern. Then, optical waveguides 12 wereformed by thermal diffusion. Thereafter, an SiO₂ film was formed on asurface on the optical waveguides 2. Modulation electrode patterns 13were formed thereon. For input and output of a laser beam, the opticalwaveguide 12 was subjected to face polishing. A constant polarizationoptical fiber 3 and a single mode fiber 4 were connected to a lightincident side and a light outgoing side, respectively. Those elementsthus manufactured had the optical modulation characteristics asillustrated in FIG. 2 and described in the foregoing. Selection was madeof an optimum one as an optical electric field sensor. Thereafter,antennas 2 for electric field detection were connected to modulationelectrodes 13. An entire arrangement was housed in the quartz package 7.The package was made up by the use of an organic adhesive and sealed sothat the internal element is not affected by an outside air. For thesake of comparison, a conventional optical electric field sensor wasprepared by the use of a package made of acrylic plastic. It was notedhere that those elements were selected to have a same optical modulationcharacteristic.

In order to demonstrate the effect of this invention, each element waswrapped by acrylic sponge and left at the room temperature for one daywith the electrodes short-circuited, so as to generate an electrostaticfield by the package. Then, the optical modulation characteristic ofeach optical electric field sensor was measured. In the conventionaloptical electric field sensor, fluctuation of the optical bias wasobserved. On the other hand, no fluctuation was found in the opticalelectric field sensor of this invention. With respect to variation ofthe ambient temperature, no fluctuation of the optical bias was observedin the optical electric field sensor of this invention using the quartzpackage even in a condition (10° C. higher than the room temperature)where fluctuation of the optical bias is caused in the conventionaloptical electric field sensor with the package made of acrylic plastic.

A similar comparison was made as regards another optical electric fieldsensor according to this invention. In this optical electric fieldsensor, the quartz package was subjected to the abrasion process. Inthis event, a more excellent result was obtained as compared with theabove-mentioned case. With the package made of ceramics, a similarresult was obtained as in case of quartz.

By the use of the package made of glass (quartz) or ceramics, theabove-mentioned optical electric field sensor according to the secondembodiment of this invention achieves stable characteristics withoutvariation of the optical modulation characteristic after assembling andwithout fluctuation of the optical bias in response to the temperaturevariation from the room temperature. Because heat insulation of theelement is stable in the optical electric field sensor according to thefirst embodiment of this invention, no consideration is required of theinfluence of the temperature drift as far as it is used at the roomtemperature (mainly used in an EMC measurement within a radio dark room,which is carried out in an environment of the room temperature). It istherefore possible to provide an optical electric field sensormanufactured with a high productivity by removing a process for makingany way against the temperature drift.

By the use of such a technique in the second embodiment of thisinvention, no electrostatic field is generated by the package materialand the influence of the variation of the ambient temperature uponvariation of the temperature within the package is reduced.

Third Embodiment

FIGS. 5(a)-5(c) show an electric field sensor according to a thirdembodiment of this invention. As illustrated in FIG. 5(a), an opticalbranched waveguide type interferometer is prepared on an LiNbO₃substrate by diffusion of Ti. One of two branched optical waveguides isprovided with electrodes. This optical modulator is fixedly housed in aglass case 1. The electrodes of the optical modulator are connected toantennas 2. A polarization maintaining fiber 3 and a single mode fiber 4are connected to a light incident side and a light outgoing side of theoptical modulator, respectively. A combination of those components formsthe optical electric field sensor. In FIG. 5(a), the optical electricfield sensor is put in a condition where the ambient temperature of theoptical electric field sensor is 30° C. A light beam is incident to thepolarization maintaining fiber 3 and passes through the optical electricfield sensor to be emitted from the single mode fiber 4 as a normaloutgoing light beam having a waveform C. An abscissa and an ordinaterepresent an applied electric field and a light intensity, respectively.Herein, adjustment is made so that the light intensity is located at amiddle point between the maximum level and the minimum level when theelectric field applied to the antenna is equal to 0 (V). The foregoingstructure is similar to that of the first embodiment. The opticalelectric field sensor according to the third embodiment of thisinvention is different from the first embodiment in that it is coveredby expanded polystyrene foam 5 having a heat insulation effect.

In the optical electric field sensor according to the third embodimentof this invention, a light beam is incident to the polarizationmaintaining fiber 3 and passes through the optical electric field sensorto be emitted from the single mode fiber 4 as a normal outgoing lightbeam having a waveform C as illustrated in FIG. 5(b). Adjustment of azero point is carried out so that the light intensity is located at amiddle point between the maximum level and the minimum level when theelectric field applied to the antenna is equal to 0 (V), as is similarto the conventional case. However, as illustrated in FIG. 5(c), adifference exists in that a waveform D of the outgoing light beamemitted from the single mode fiber 4 is coincident with the waveform Cwithout being subjected to the temperature drift. Specifically, in thewaveform D, the light intensity is equal to 0 (V) when the electricfield applied to the antennas is equal to 0 (V). Thus, the waveform isnot varied from the waveform C of the normal outgoing light beam. Asdescribed, the expanded polystyrene foam 5 suppresses the temperaturedrift (see FIG. 1(c)) of the optical electric field sensor to achieve anexcellent temperature characteristic and a stable sensitivity.

From the above-mentioned result, it is understood that the opticalelectric field sensor using a material having a heat insulation effectaccording to the third embodiment of this invention can suppress theinfluence of the temperature drift resulting from variation of theambient temperature to improve a temperature characteristic and to keepa stable sensitivity of the sensor. According to the third embodiment ofthis invention, the influence of variation of the ambient temperature ofthe optical electric field sensor upon the optical crystal of theoptical modulator is suppressed to be small. It is therefore possible toprovide an optical electric field sensor having an excellent temperaturecharacteristic. According to the third embodiment of this invention, itis possible not only to contribute to the improvement of the temperaturecharacteristic of the optical electric field sensor, which is a problemin prior art, but also to provide an optical electric field sensormanufactured at a relatively low cost in a reduced process time with ahigh productivity.

Fourth Embodiment

FIGS. 6(a) and 6(b) show a main portion of an optical electric fieldsensor according to a fourth embodiment of this invention. Asillustrated in FIGS. 6(a) and 6(b), a Ti (film thickness of 800 A)thermal diffusion optical waveguide (hereinafter simply called anoptical waveguide) 12 was formed on a X-cut substrate 11 of LiNbO₃crystal as a branch interference type optical waveguide which isbranched and again joined together. Modulation electrodes 13 werearranged in an area after being branched and before joined again. Thus,a Mach-Zehnder optical interferometer 20 was manufactured. On oppositeends of the element, two short dipole antennas (not shown) of 75 mm wereconnected to the modulation electrodes 13 in the manner similar to FIG.2(a). A constant polarization optical fiber and a single mode opticalfiber were connected to a light input side 15 and a light output side16, respectively (not shown). An input light beam was a laser light beamhaving a wavelength of 1.3 μm. An output light beam was subjected to O/Econversion for measurement. A direct current voltage 18 was applied tothe short dipoles of the optical electric field sensor thusmanufactured. A half-wavelength voltage V.sub.π was obtained from theintensity variation of the optical output in response to the appliedvoltage.

By the use of a conductive spray of this invention, as shown in FIG.6(b), a conductive resin film 14 was formed (process time being on theorder of 5 seconds per one) on the optical modulator portion of theoptical electric field sensor thus manufactured. In a thermostaticchamber, variation of the intensity of the outgoing light beam inresponse to temperature variation was confirmed. The temperature wasvaried stepwise by 10° C. in a range between -10° C. and 60° C. For thesake of comparison, a similar test was carried out for the opticalelectric field sensor without treatment by the conductive spray. Thetest data were dealt with in the form of voltage shift which is obtainedfrom the light intensity with reference to an SG curve 25 in FIG. 7 andnormalized by the half-wavelength voltage V.

The optical electric field sensor without using the conductive spray wasunstable in light intensity, the level of which fluctuated even around25° C. With respect to the temperature variation, a shift not smallerthan the half-wavelength voltage was confirmed. On the other hand, theoptical electric field sensor according to this invention exhibited novariation in light intensity even around 25° C. It was confirmed thatthe shift in the optical bias is not greater than ±0.3% (normalized bythe half-wavelength voltage) in an environment of the temperaturebetween -10° C and 60° C.

The above-mentioned result shows that the fourth embodiment of thisinvention is effective in improvement of the temperature characteristicof the optical electric field sensor. Also from the fourth embodiment,it is confirmed that this invention greatly contributes to improvementof productivity because manufacture is easily carried out at a very lowcost as compared with the conventional case. Accordingly, it is foundout that the fourth embodiment of this invention provides an opticalelectric field sensor with a conductive film formed by an inexpensiveand simple process.

Fifth Embodiment

FIGS. 8(a) and 8(b) show a main portion of an optical electric fieldsensor according to a fifth embodiment of this invention. As illustratedin FIGS. 8(a) and 8(b), a Ti (film thickness of 800 A) thermal diffusionoptical waveguide 12 was formed on a X-cut substrate 11 of LiNbO₃crystal as a branch interference type optical waveguide which isbranched and again joined together. Modulation electrodes 13 were formedin an area after branched and before joined again. Thus, a Mach-Zehnderoptical interferometer was manufactured in the manner similar to thefourth embodiment. In the fifth embodiment, silicone 17 was applied ontoa region including the modulation electrodes 13 adjacent to the opticalwaveguide 12. On opposite ends of the element, two short dipole antennasof 75 mm were connected to the modulation electrodes 13. A constantpolarization optical fiber and a single mode optical fiber (not shown)were connected to a light input side and an output side, respectively.An input light beam was a laser light beam having a wavelength of 1.3μm. An output light beam was subjected to O/E conversion formeasurement. A direct current voltage was applied to the short dipolesof the optical electric field sensor thus manufactured. Ahalf-wavelength voltage V.sub.π was obtained from the intensityvariation of the optical output in response to the applied voltage. Theresult was illustrated in FIG. 9. The optical electric field sensor thusmanufactured was applied with a DC voltage (12 V), which tends to causecharacteristic deterioration, and left for 100 hours. Then, the DC driftwas measured. Likewise, the sample without being applied with siliconewas measured for comparison.

After measurement, a test was carried out for 100 hours at a constanttemperature and humidity of 60° C. and 60% which was a bad environmentrealized by the use of common tap water for humidification. Then, the DCdrift was similarly measured. As a result, the time period until theinitial DC drift is caused was unchanged in the sample applied with thesilicone. On the other hand, the time period was reduced more than 10times in the sample without being applied with the silicone. Accordingto the fifth embodiment of this invention, the agent having a stablecharacteristic, namely, having no undesired ions and a high reliability,is applied in an area between the electrodes, where any foreignsubstance tends to invade. It is therefore possible to provide anoptical electric field sensor which is capable of inhibitinginterference with an external environment to readily prevent invasion ofdirt or a foreign substance. It is confirmed that, by the use of theoptical electric field sensor according to the fifth embodiment of thisinvention, a stable optical electric field sensor is provided which cansuppress variation of the DC drift although a process is very simple. Inaddition, according to the fifth embodiment of this invention, it ispossible to provide an optical electric field sensor having acharacteristic stable for a long time, inasmuch as invasion of a foreignsubstance is avoided during application of silicone. Since no specialfacility is required and the operation itself is very easy, nosubstantial increase in process time is required.

Sixth Embodiment

FIG. 10 shows an optical electric field sensor according to a sixthembodiment of this invention. Referring to FIG. 10, a Mach-Zehnderinterferometer pattern similar to the conventional one illustrated inFIG. 2(a) was formed on a Z-cut substrate 21 of LiNbO₃ crystal by a Tipattern. Then, an optical waveguide 12 was formed by thermal diffusion.Thereafter, an SiO₂ film was formed on a surface of the opticalwaveguide 12. A pattern of modulation electrodes 22 was formed thereon.For input and output of a laser beam, the optical waveguide wassubjected to face polishing. A constant polarization optical fiber 3 anda single mode fiber 4 were connected to a light incident side and alight outgoing side, respectively. The element thus manufactured has anoptical modulation characteristic as illustrated in FIG. 3, like in theconventional case. The foregoing structure is similar to that of theconventional case. Thereafter, according to the sixth embodiment of thisinvention, antennas 2 for electric field detection were connected to themodulation electrodes 22. An entire arrangement was packaged asillustrated in FIG. 10. The antennas are not illustrated in FIG. 10. Thepackage was made up by the use of an organic adhesive and sealed so thatthe internal element is not affected by an outside air.

By the use of two kinds of vinyl chloride plates, namely, a vinylchloride plate subjected to the antistatic treatment according to thesixth embodiment of this invention and a comparative vinyl chlorideplate not subjected to any treatment, the packages were individuallymade up and subjected to measurement of electric charge. At first, theabove-mentioned two kinds of packages were applied with electric fieldsof a same intensity. After lapse of a predetermined time duration,electric charges were measured and compared with each other. As aresult, after 30 seconds from application of the electric field, nosubstantial electric charge was observed in the antistatic-treatedpackage according to the sixth embodiment of this invention. On theother hand, in the package of the comparative example without anytreatment, the electric charge was measured to correspond toapproximately a half of the intensity of the applied electric field.

The above-mentioned result shows that the optical electric field sensorusing the package according to the sixth embodiment of this invention iscapable of carrying out optical modulation of an electric field to bemeasured alone. This is because no disturbance or influence is given tothe ambient electric field by electrification of the package. Inaddition, according to the sixth embodiment of this invention, it ispossible to provide the optical electric field sensor which is capableof removing an electrostatic field from the package material afterpackaging so as not to disturb the electric field to be measured.According to the sixth embodiment of this invention, it is possible toprovide the optical electric field sensor which is capable of measuringthe electric field free from an influence of disturbance by removing theelectrostatic field generated from the package material even if theplastic material is used. According to the sixth embodiment of thisinvention, it is possible to provide the optical electric field sensorwhich has an excellent temperature characteristic, by suppressing theinfluence of the variation of the ambient temperature around the opticalelectric field sensor upon the optical crystal of the optical modulator.According to the sixth embodiment of this invention, it is possible notonly to contribute to improvement of the temperature characteristic ofthe optical electric field sensor, which has been a problem in priorart, but also to provide the optical electric field sensor manufacturedat a relatively low cost in a reduced process time with a highproductivity.

As described above, according to the first through the sixth embodimentsof this invention, it is possible to provide the optical electric fieldsensor having a structure of suppressing heat conduction of thermalfluctuation outside of the optical electric field sensor to the opticalcrystal so that any device of a high accuracy is unnecessary and thetemperature characteristic is improved.

Industrial Application Field

As described above, the electric field sensor according to thisinvention is adapted to measurement of an electric field intensitywithin a spatial field, typically, EMC measurement (noise measurement).

We claim:
 1. An optical electric field sensor which comprises:optical components including an optical crystal and which is for measuring an intensity of an electric field, spontaneously or forcedly generated, by the use of variation of at least one of an intensity, a phase, and a polarization direction of a light beam passing through said electric field; and wherein said optical components are arranged in a package made of a first heat insulation material.
 2. An optical electric field sensor as claimed in claim 1, wherein said first heat insulation material is at least one material selected from a ceramics material and a glass material.
 3. An optical electric field sensor as claimed in claim 2, wherein a main portion of a surface of said package is subjected to an abrasion treatment.
 4. An optical electric field sensor as claimed in claim 1, wherein said electric field is applied through antennas connected to said optical crystal.
 5. An optical electric field sensor as claimed in claim 1, wherein said package is fixedly surrounded by a second heat insulation material.
 6. An optical electric field sensor as claimed in claim 5, wherein said second heat insulation material comprises expanded polystyrene foam.
 7. An optical electric field sensor as claimed in claim 1, wherein:said optical components are arranged such that at least a pair of modulation electrodes are located in the vicinity of an optical waveguide formed on an optical crystal substrate having an electrooptical effect; and an electric field spontaneously or forcedly generated is led to said pair of modulation electrodes.
 8. An optical electric field sensor which comprises:at least a pair o modulation electrodes located in the vicinity of an optical waveguide formed on an optical crystal substrate having an electrooptical effect and which is for measuring an electric field intensity by use of variation of at least one of an intensity, a phase, and a polarization direction of a light beam that is caused by leading to said pair of modulation electrodes an electric field spontaneously or forcedly generated; and wherein at least one of treatments is carried out which include application of conductive resin onto an entire surface of said optical crystal substrate and application of silicone between said modulation electrodes.
 9. An optical electric field sensor as claimed in claim 8, wherein said optical crystal substrate is fixedly surrounded by a heat insulation material.
 10. An optical electric field sensor which comprises:optical components including an optical crystal and which is for measuring an intensity of an electric field, spontaneously or forcedly generated, by use of variation of at least one of an intensity, a phase, and a polarization direction of a light beam passing through said electric field; and wherein said optical components are arranged in a package made of a plastic material having an antistatic-treated surface.
 11. An optical electric field sensor as claimed in claim 10, wherein said plastic material comprises vinyl chloride.
 12. An optical electric field sensor as claimed in claim 10, wherein a main portion of a surface of said package is subjected to an abrasion treatment.
 13. An optical electric field sensor as claimed in claim 10, wherein said electric field is applied through antennas connected to said optical crystal.
 14. An optical electric field sensor as claimed in claim 10, wherein said package is fixedly surrounded by a heat insulation material.
 15. An optical electric field sensor as claimed in claim 14 wherein said heat insulation material comprises expanded polystyrene foam.
 16. An optical electric field sensor as claimed in claim 10, wherein:said optical components are arranged such that at least a pair of modulation electrodes are located in the vicinity of an optical waveguide formed on an optical crystal substrate having an electrooptical effect; and an electric field spontaneously or forcedly generated is led to said pair of modulation electrodes. 